1. Field of the Invention
This invention relates to methods and apparatus for determining the critical dimension of workpieces. 2. Description of Related Art
An occasional problem with a conventional automated Critical Dimension Scanning Electron Microscope (CD SEM) measurement is poor correlation thereof with subsequent electrical measurements. This problem can be due to feature positions to be measured, for example, the foot of a photoresist line being obscured by an overhanging structure. Other examples are T-topping, undercutting, and negative angle or recursive sidewall.
Currently available standard top/down CD SEM systems prevent the SEM electron beam (SEM-beam) from tilting relative to the sample for several reasons. However, at least one CD SEM provider is developing a system that can quickly and automatically tilt the beam by several degrees and acquire secondary electron waveforms or images from scanning the same structure at various tilt angles.
Such technology can advance the core capability of the CD SEM only if the additional information resulting from changing the angle of deflection of the scanning SEM-beam can be used quickly and in an automated fashion to improve the accuracy of measurement. Problems needing to be solved include positional alignment of the waveforms, separating various contributors to the effective edge width of a tilted structure, and finally, synthesizing the information into a critical dimension measurement.
There has been considerable effort directed at extracting three-dimensional information from two images acquired at different angles of view (stereoscopic imaging), as in robotic vision.
These methods use the phenomena of shadowing and parallax to calculate the relative coordinates (including height) of identifiable features in two or more images. Unfortunately, on the scale of interest for CD metrology (nanometers) and for the primary structures of interest (straight lines or spaces), there are few dependable identifiable features. More seriously, the SEM-beam interaction with the structure is very different from the interaction physics of these other applications. Successful sidewall metrology needs to account for the finite size of the SEM-beam and the interaction volume within the structure material.
An example of scatterometry is found in the area of semiconductor manufacturing metrology. In the approach recently commercialized by Biorad, a defocussed laser beam scatters off of a periodic array of structures on the wafer (target) and the zeroth order diffracted beam intensity is measured for two polarizations of light. Data is collected as a function of the incident angle. The resulting waveform is compared with simulations. The ability and resources for calculating the electromagnetic response for model structures is crucial to this approach. Other variations on this approach include using higher order diffracted beams or multiple wavelengths of light. None of these methods deals with images or waveforms acquired by scanning focused SEM-beams or the very different interaction physics of an SEM-beam with matter.
One noteworthy approach to improving the accuracy of top/down CD SEM metrology is the work of the Spectel Corporation. System responses, based on the use of an approximate simulation of the SEM-beam interaction with model structures, produce a database of waveforms. The best match to the actual waveform is used to interpret the measurement. That is similar in concept to the commercialized scatterometry approach. Possibly, this approach can be applied to the tilted SEM-beam CD SEM system. However, the overhead in calculation resources is significant and the accuracy of the modeling, especially in the presence of sample charging, is highly questionable.
Beam tilting is the same thing as beam deflection that are used for column alignment as exemplified by U.S. Pat. No. 6,066,849 of Masnaghetti et al. for xe2x80x9cScanning Electron Beam Microscopexe2x80x9d which applies an x tilt voltage and a y tilt voltage but as described at Col. 11, lines 41-53 , it is employed as follows:
xe2x80x9cThe upper quadrupole . . . is configured to align the beam after a particular gun lens voltage is selected. In other words, the beam may have to be moved such that it is realigned with respect to the aperture. This realignment is accomplished by supplying an X and Y tilt voltage from the multiplexer control system . . . and the beam may be realigned with respect to the aperture by setting the X and Y tilt voltage values that are supplied to the upper quadrupole . . . xe2x80x9d
See U.S. Pat. No. 5,969,273 of Archie et al. xe2x80x9cMethod and Apparatus for Critical Dimension and Tool Resolution Determination Using Edge Widthxe2x80x9d describes measuring hump width to obtain SEM resolution information. See U.S. Pat. No. 6,025,600 of Archie et al. xe2x80x9cMethod for Astigmatism Correction in Charged Particle Beam Systemsxe2x80x9d; and U.S. Pat. No. 5,869,833 of Richardson et al. xe2x80x9cElectron Beam Dose Control for Scanning Electron Microscopy and Critical Dimension Measurement Instrumentsxe2x80x9d.
A common prior art algorithm is to declare the outer extremal slope location for each edge of a feature on a sample to be the location of the edge and therefore to report the distance between these locations.
The discovery of a simple relationship between structure properties and the Distance between Extremal Slope Locations (DESL) in SEM data as a function of electron beam tilt angle forms the basis for an automated methodology of obtaining such structural properties without the need for extraordinary alignment or 3D (three-dimensional) reconstruction techniques. Normally careful alignment of the data is required to extract three-dimensional information from multiple SEM images or waveforms (one-dimensional digitized line scans). In cases of interest, related to this invention, that alignment is on the nanometer scale. Today, it is not possible to acquire multiple SEM images or waveforms on the nanometer scale, after stage movement, with blind navigation. Use of pattern recognition can improve matters, if suitable pattern recognition targets that are required are available, which is generally not true.
This invention gets around the alignment problem by not requiring alignment. Instead, each waveform (either directly obtained from the SEM or extracted from a SEM image) can be analyzed to find the locations of extremal slopes for each structural edge of interest. The DESL value so determined should have a precision of a few nanometers. With calibration, the accuracy of the DESL value measured should be comparable to its precision.
In order to automate a fast determination of structure properties (height as well as, left and right sidewall angles), the method of this invention minimizes the actual number of measurements in real time by requiring that for each feature edge, there are two DESL values determined at different tilt angles that are larger than a threshold value, which is set at the time of calibration.
Variations of this include requiring only one DESL measurement greater than the threshold value if either sidewall angle or structure height is already know. Another variation is to use the height determined from one edge analysis in the analysis for the other edge. This then requires two DESL measurements above the threshold value for one side but only one DESL measurement above the threshold value for the other side. Another variation, is to allow the gathering of additional DESL information beyond the minimum necessary in order to improve measurement uncertainty or to perform consistency checking.
The analysis of the DESL information as a function of tilt angle is as follows: Provided all the DESL values being used for one edge (two or more) are greater than the threshold value, then the DESL values [nm] versus tilt angle [radians] are fitted to a straight line. The important properties of the straight line are the slope and the Y-axis intercept. Because the cases of practical interest have the sidewall angle and the beam tilt angles small (less than 10 degrees), tan xcex8=xcex8 is a good approximation. If applied to a situation with larger angles, the modification of the method is straightforward. In the small angle case, the slope determined from the straight-line-fit is the structure height in nanometers. The Y-axis intercept determined from the linear regression determines the sidewall angle by the following formula:
In a situation where one of the DESL values to be used is close to the value of the sum of K0+K determined during calibration, then a more accurate, non-linear analysis is necessary. The DESL and tilt angle values will be fit to the following functional form with the structure height H and sidewall angle SA being the fitting parameters:       DESL    =                                                      [                              H                *                                  tan                  ⁡                                      (                                                                  φ                        0                                            +                                              ϕ                        e                                                              )                                                              ]                        2                    +                      K            0            2                              +      K                          where                                                  H            =                          structure              ⁢                              xe2x80x83                            ⁢              height                                ,                                                                        φ              0                        =                          sidewall              ⁢                              xe2x80x83                            ⁢              angle              ⁢                              xe2x80x83                            ⁢              deviation              ⁢                              xe2x80x83                            ⁢              from              ⁢                              xe2x80x83                            ⁢              vertical                                ,                                                          ϕ            e                    =                      tilt   of   the   SEM-beam,                                                        DESL          =                      Distance            ⁢                          xe2x80x83                        ⁢            between            ⁢                          xe2x80x83                        ⁢            Extermal            ⁢                          xe2x80x83                        ⁢            Slope            ⁢                          xe2x80x83                        ⁢            Locations                                                                    K            0                    =                      constant            ⁢                          xe2x80x83                        ⁢            determined            ⁢                          xe2x80x83                        ⁢            during            ⁢                          xe2x80x83                        ⁢            calibration                                                        K          =                      constant            ⁢                          xe2x80x83                        ⁢            determined            ⁢                          xe2x80x83                        ⁢            during            ⁢                          xe2x80x83                        ⁢            calibration                              
Once the structural properties of height H and left and right sidewall angles LSA and RSA are determined, these can be used to determine a more accurate value for the Critical Dimension of the structure. Often times the structure being measured is made of photoresist patterned by lithography. This photoresist pattern will be used in a subsequent processing step as a mask. For processes like isotropic etching, ion diffusion, and plating, the base width of the structure defines the extent of this subsequent processing step and therefore is the Critical Dimension needing to be measured. So in cases such as these and more, the measurement of greatest value is the structure Base Width BW. In other cases, such as a subsequent anisotropic processing step, the Maximum Structure Width MSW anywhere from structure top to base is most important. There is a need for flexibility in the use of the additional structural properties of height H and sidewall angle SA since the critical dimension CD depends not only on the actual structure but upon the application of this structure to a subsequent processing step.
In the case where the Base Width is the Critical Dimension, prior art algorithms which only use untilted electron beam information, are usually adequate provided the base is not obscured because of a Negative Sidewall Angle. As stated above a common prior art algorithm is to declare the outer extremal slope location for each edge to be the location of the edge and therefore report the distance between these locations without taking into account that one or both the edges of the structure may be obscured by a tilted Negative Sidewall Angle with the corner above the edge obscuring the sidewall and the feature to be detected. Therefore, the first use of the new information is to confirm or deny the applicability of the prior art algorithm. If either sidewall has a Negative Sidewall Angle, then either a correction must be made to the prior art algorithm result or a totally new algorithm which uses all the structural information should be used. While this decision making and calculation could be made by a host computer once all the information has been sent from the CDSEM, it is preferred for real time reporting that the CDSEM computer actually do this processing.
Assuming that the prior art algorithm in the case of a negative sidewall angle is actually finding the location of the edge only at the top of the structure, then the correction that must be added to correctly determine the edge base location is H tan(SA) where H is the structure height and SA the sidewall angle. This correction should be applied to both edges if both have negative sidewall angles. So in general, if CD stands for the Critical Dimension to be reported and CDO stands for the result from the prior art algorithm, then perform the calculations as follows:
The flexibility needed to handle multiple prior art algorithms and multiple definitions of the critical dimension can be achieved by allowing the user to choose an appropriate value for the constant K2 in the following modified version of the calculation:
In accordance with this invention, a method/system/apparatus for making Scanning Electron Microscope (SEM) scans of a workpiece comprises:
(a) a method/means for setting an SEM beam to a first/next deflection tilt angle,
(b) a method/means for scanning of a region of the workpiece at the deflection tilt angle to acquire a waveform,
(c) a method/means for analyzing a waveform to determine a DESL value for each edge of interest,
(d) a method/means for determining whether there is sufficient information for each structural edge and if NO returning to step (a) and if YES proceeding to step (e),
(e) a method/means for determining height and sidewall angle values for each structural edge, and
(f) a method/means for reporting the height and sidewall angle for each structural edge.